Copper composites directly bonded to ceramics

ABSTRACT

A ceramic member (52) is direct-bonded to a copper composite substrate by heating to diffuse copper to the surface of the copper composite substrate (56), oxidizing the copper composite substrate following heating, placing a ceramic member in contact with the resulting oxidized substrate, and forming a copper-copper oxide eutectic (54) at the interface between the copper composite substrate and the ceramic member by heating. The eutectic, upon cooling, forms a bond between the copper composite and the ceramic.

This is a continuation division of application Ser. No. 08/268,488 filedJun. 30, 1994, now U.S. Pat. No. 5,490,627.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention reiates to the bonding of copper composites tononmetallic materials. In one of its more particular aspects thisinvention relates to the direct bonding of copper composites toceramics.

2. Description of Related Art

In the design of electronic circuits, a principal consideration is theneed to control temperature in order to improve reliability of thecircuit components and to meet circuit performance requirements. Inparticular, in the case of power hybrids, which may have power densitiesexceeding 5 watts/in², it is essential to provide a heat sink, or heatspreader, such as a metal layer, to remove a considerable proportion ofthe heat generated in the power hybrid by the active and passivedevices. Similar considerations are applicable to other microcircuitssuch as standard hybrids and multi-chip modules.

Copper layers have found wide use as heat sinks. Typically, they havebeen bonded to ceramic members by soldering or brazing. A disadvantageof such bonding techniques is that they introduce an additional thermalinterface between the heat sink and ceramic member. Voids may also occurduring the soldering or brazing process which adversely affects thermalconductivity.

U.S. Pat. No. 3,766,634, in an attempt to eliminate such interfaces andvoids, describes the direct bonding of copper and other metals tononmetallic substrates. A copper-copper oxide eutectic is used to bondcopper to an alumina substrate, for example.

U.S. Pat. No. 4,563,383 describes a direct bond copper ceramic substratefor use in high temperature thick film processing. The substrateconsists of two outer ceramic layers and a central copper core formed ofthree layers bonded by a copper oxygen eutectic to the outer ceramiclayers. By bonding ceramic to both sides of the copper core, stresssymmetry is said to be provided.

One problem encountered in the provision of direct bond metal ceramicstructures is that there is a marked temperature coefficient ofexpansion mismatch between the metal and the ceramic. For example, purecopper has a coefficient of expansion of 16.8 ppm/°C. while alumina,beryllia and aluminum nitride, which are typical ceramic materials usedin microcircuit packaging, have coefficients of expansion ranging fromabout 4.3 to 7.0 ppm/°C. Because of this mismatch, the thickness of thecopper layer is severely limited. For example, with a 0.025-inch thickceramic substrate, the thickness of the copper layer is limited toapproximately 0.012 inch, because thicker copper layers would result inconsiderable warping of the resulting structures.

Thicker copper layers, however, are desirable to attach the copperceramic material to a base structure, since the ceramic is too brittleto allow attachment by screwing down to the base structure.

In an effort to avoid the adverse effects of the mismatch ofcoefficients of expansion of metals and ceramics, approximately equalamounts of copper have been provided on both the top and bottom sides ofthe ceramic substrate to balance out the stresses. The thickness of suchcopper layers can be approximately one-tenth to one-third the substratethickness. For example, with a 0.025 inch thick substrate, each copperlayer might be approximately 0.008 inch thick, which is too thin toprovide sufficient rigidity for attachment to a base structure.

It would be desirable to provide heat sinks for hybrids and multi-chipmodules having sufficient rigidity to allow screwing down the heat sinkto a base structure. It would also be desirable to avoid the need forusing amounts of the metal layers on both sides of the ceramicsubstrate.

Accordingly, it is an object of this invention to provide a heat sinkdirectly bonded to a ceramic member, which heat sink is of sufficientrigidity to be readily attached by screwing down to a base structure.

Another object of this invention is to provide metal ceramic structuresin which the metal and ceramic have similar temperature coefficients ofexpansion.

A further object of this invention is to provide a process for bondingtogether a heat sink and a ceramic member without requiring soldering orbrazing.

Other objects and advantages of this invention will become apparent inthe course of the following detailed disclosure and description.

SUMMARY OF THE INVENTION

The present invention utilizes a copper composite substrate which isdirectly bonded to a ceramic member. Standard copper composites cannotbe directly bonded to ceramics as there is insufficient copper on thesurface. In this invention, the copper composite substrate is subjectedto a temperature sufficient to cause diffusion of copper to a regionadjacent the surface of the composite. Oxidation of the diffused copperprovides sufficient oxide to form a copper-copper oxide eutectic betweenthe ceramic member and the copper composite upon heating. By placing aceramic member in contact with the copper composite following oxidationand heating in an inert atmosphere, the copper-copper oxide eutecticwhich forms wets the surfaces of the oxidized copper composite and theceramic so that, upon cooling, a strong bond is produced.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is an enlarged diagrammatic cross-section of a copper-aluminastructure according to the prior art which utilizes copper layers oneither side of a single alumina layer;

FIG. 2 is an enlarged diagrammatic cross-section of a copper-aluminumnitride structure according to the prior art which utilizes copperlayers on either side of an aluminum nitride layer separated from thecopper layer by aluminum oxide layers; and

FIG. 3 is an enlarged diagrammatic cross-section of a copper-compositeceramic structure according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to overcome some of the deficiencies of the prior art, a coppercomposite, rather than pure copper, is used as the heat sink material.By using a copper composite having a coefficient of expansion close tothat of the ceramic which partially overlies the heat sink, such ascopper-tungsten or copper-molybdenum, it is possible to provide arelatively thick layer of copper composite which can be drilled andscrewed down to a base structure. For example, a copper-tungstencomposite which contains 75% tungsten and 25% copper has a coefficientof expansion of 6.0 ppm/°C., which is close to that of the ceramic. Thecoefficients of expansion of alumina, beryllia, and aluminum nitride,for example, fall in the range of 4.3 to 7.0 ppm/°C. Although thethermal conductivity is reduced from 401 watts/°C.-meter for pure copperto 287 watts/°C.-meter for a 75-25 mixture of tungsten and copper, thereduced thermal conductivity is sufficiently high for most heat sinkapplications.

A typical previously available copper ceramic structure is illustratedin FIG. 1 wherein the numeral 10 represents the overall structure, whichconsists of a layer of alumina 12 with copper-copper oxide eutecticlayers 14 and 16 attached thereto. Attached to the eutectic layers arecopper layers 18 and 20.

FIG. 2 illustrates a similar previously used structure in which theceramic is aluminum nitride, which requires growing an interveningaluminum oxide layer to which are applied the eutectic and copper layerssince aluminum nitride cannot be directly bonded to copper. In FIG. 2,numeral 30 represents the overall structure, which consists of aluminumnitride layer 32 sandwiched between aluminum oxide layers 34 and 36.Copper-copper oxide eutectic layers 38 and 40 are adjacent to aluminumoxide layers 34 and 36. The eutectic layers 38 and 40 separate thealuminum oxide layer from the copper layers 42 and 44. As shown in FIGS.1 and 2, dual copper layers were previously used in order to preventwarping of the structure due to the mismatch in coefficients ofexpansion of the copper and alumina or aluminum nitride. The structureof the present invention differs from previously used structures in thata copper composite such as copper-tungsten is utilized as the heat sinkin the present invention.

FIG. 3 illustrates the present invention, wherein the numeral 50represents the overall structure with ceramic member 52 being adjacentcopper-copper oxide eutectic layer 54, which in turn is adjacent to adiffused copper layer 56 which forms a part of copper-tungsten or othercopper composite layer 58.

The copper-tungsten or other copper composite heat sink assembly of thepresent invention is readily produced by heating the copper-tungstensubstrate of the desired thickness to a temperature at which copperdiffuses into a region adjacent to the surface of the copper-tungstensubstrate. A temperature of at least about 1200° C. has been found to besuitable for this purpose. The next step is oxidizing the diffusedcopper to produce copper oxide at the surface of the copper-tungstensubstrate. The resulting oxidized substrate is next placed in contactwith a ceramic member in an inert atmosphere and subjected to atemperature higher than the copper-copper oxide eutectic temperature andlower than the melting point of copper to produce a copper-copper oxideeutectic at the interface between the copper-tungsten substrate and theceramic member. Temperatures in the range of about 1065° C. to 1083° C.have been found suitable for this step. The next step is cooling tosolidify the eutectic and form a tenacious bond between thecopper-tungsten and the ceramic. If necessary or desired, the resultingcoppertungsten heat sink assembly can be cleaned and plated to preventfurther oxidation.

In addition to copper-tungsten composites which are commerciallyavailable in several different formulations such as 90/10, 85/15, 80/20and 75/25, copper/tungsten from Polese Company, Inc. of San Diego,Calif.; Amitek of Pennsylvania; and Sumitomo of Japan; other coppercomposites can be used. As previously mentioned, copper-molybdenumcomposites can be similarly used.

Ceramics other than alumina, beryllia and aluminum nitride can also beused. For example, fused silica, titanates and spinels are satisfactoryfor this purpose.

The copper composite-ceramic bonds of the present invention are usefulin a variety of semiconductor applications. For example, they may finduse in the fabrication and assembly of hybrid circuits where a tenaciouspermanent bond is required. They may be used wherever it is required toprovide a heat sink for an electronic device which utilizes a ceramicsubstrate.

Having thus described exemplary embodiments of the present invention, itwill be understood by those skilled in the art that the withindisclosure is exemplary only and that the present invention is not to belimited in scope except in accordance with the following claims.

What is claimed is:
 1. A metal-ceramic structure comprising:a coppercomposite substrate having a diffused copper region adjacent the surfacethereof, a copper-copper oxide eutectic layer formed upon saidsubstrate, and a ceramic member bonded to said copper-copper oxideeutectic layer.
 2. The structure of claim 1 wherein the copper compositeis selected from the group consisting of copper-tungsten andcopper-molybdenum.
 3. The structure of claim 1 wherein the coppercomposite is copper-tungsten.
 4. The structure of claim 1 wherein theceramic is selected from the group consisting of alumina, beryllia, andaluminum nitride.